The present invention relates to a plastic screw grommet for connecting and securing an attached component such as a cover to a workpiece such as a car body by screwing in a tapping screw and, more specifically, to a screw grommet that maintains a high level of attachment force while reducing the fastening torque load on the legs of the shank when the tapping screw is screwed in.
A screw grommet, in which the grommet is equipped with a flange and a shank extending from the flange, in which the shank is hollow and inserted into an attachment hole in a workpiece, in which the shank has flexible legs extending towards the flange from the end of the shank and extending radially outward from the shank with the end towards the flange being the free end, in which a threaded through-hole is formed in the flange and shank for receiving a tapping screw, and in which screwing the tapping screw into the threaded through-hole prevents warping of the free end of the legs towards the inside radially are well known in the art. Specific examples are disclosed in Japanese Unexamined Patent Application Disclosure [Kokai] No. 55-14359, Japanese Unexamined Utility Model Application Disclosure No. 62-200807, Registered Design No. 704108 (disclosed in 1987), and Kokai No. 5-187422. The screw grommets disclosed in these publications all have threaded sections in the ends of the shank legs for receiving a tapping screw and sections near the workpiece adjacent to the flange for avoiding the tapping screw. For this reason, the force applied by the legs to the work-piece is somewhat weak. A screw grommet was disclosed in Kokai No. 1-216109 that does not rotate with the tapping screw even when applied to a round attachment hole. In this screw grommet, the central section of the legs in the shank expand outward.
The legs expand outward strongly when the tapping screw is screwed into the free end of the legs to secure the screw grommet to the workpiece and keep it from rotating with the tapping screw.
Because the screw grommet disclosed in Kokai No. 1-216109 is attached strongly to the workpiece, it is ideal. However, when the tapping screw is screwed into the legs and strong fastening torque from the tapping screw is applied to the free end of the legs, strong moment is applied to the base of the legs (end of the shank), and the legs are twisted and damaged. Because the latest fastening tools are strong and compact, the tapping screw applies strong torque, and the probability of damage to the legs is great.
The purpose of the present invention is to provide a screw grommet in which the legs of the shank expand outward strongly to secure the grommet to the workpiece while preventing damage to the legs due to fastening torque from the tapping screw.
Another purpose of the present invention is to provide a screw grommet in which the legs of the shank expand outward strongly to bring the legs into contact with the workpiece and secure the workpiece even more firmly.
The purposes of the present invention are achieved by providing a screw grommet wherein the grommet is equipped with a flange and a shank extending from the flange, wherein the shank is hollow and inserted into an attachment hole in a workpiece, wherein the shank has flexible legs extending towards the flange from the end of the shank and extending radially outward from the shank with the end towards the flange being the free end, wherein a threaded through-hole is formed in the flange and shank for receiving a tapping screw, wherein screwing the tapping screw into the threaded through-hole prevents warping of the free end of the legs towards the inside radially, wherein the flange and the legs securely in contact with the edge of the attachment hole in the workpiece fix the grommet to the workpiece, wherein a threaded section is formed on the inside section of the free end of the legs for receiving the tapping screw and extending towards the inside so the diameter of the threaded through-hole becomes smaller than the outer diameter of the tapping screw, wherein a tube-shaped fastening torque bearing section extending opposite the shank is integrated with the flange, wherein a threaded through-hole is formed in the fastening torque bearing section with a diameter smaller than the outside diameter of the tapping screw for receiving the tapping screw, and wherein the fastening torque from screwing in the tapping screw is born by the fastening torque bearing section outside the threaded section of the legs.
Because the screw grommet has a threaded section on the inside of the free ends of the legs with a diameter greater than the outer diameter of the tapping screw, the free ends of the legs expand outward strongly and the screw grommet is secured to the workpiece firmly. Because a threaded through-hole with a diameter smaller than the tapping screw is formed in the fastening torque bearing section and the fastening torque from screwing in the tapping screw is born by the fastening torque bearing section outside the threaded section of the legs, strong fastening torque is not concentrated in the legs and the legs are less likely to be damaged. As a result, a fastening tool with high fastening torque can be used and the screw grommet can be used to quickly attach a component.
Ideally, the screw grommet should be designed so the legs are formed as pairs of anchor-shaped legs opposed to each other diametrically, the ends of the legs are connected to the main section of the shank and the legs are separated from the main section of the shank by a U-shaped groove, and ribs are formed extending axially towards the threaded through-hole on the inside of the main section of the shank separate from the legs for bearing some of the fastening torque when the tapping screw is screwed in. As a result, high fastening torque can be dispersed. The circumferential width of the shank in the U-shaped groove becomes smaller, and the twisting of the legs due to fastening torque when the tapping screw is screwed in can be prevented by contact with the main body of the shank. A locking shoulder is formed in the anchor-shaped legs to engage the edge of the hole in the workpiece, and the locking shoulder is not parallel but at a constant angle to the engaged surface of the workpiece. As a result, the workpieces of varying thicknesses can be secured firmly. Ideally, a holding clip with a U-shaped curve substantially parallel to the flange surface is integrated into the end of the fastening torque bearing section. This holding clip can be used to connect different components.
The present invention is also a screw grommet, in which the grommet is equipped with a flange and a shank extending from the flange, in which the shank is hollow and inserted into an attachment hole in a workpiece, in which the shank has flexible legs extending towards the flange from the end of the shank and extending radially outward from the shank with the end towards the flange being the free end, in which a threaded through-hole is formed in the flange and shank for receiving a tapping screw, in which screwing the tapping screw into the threaded through-hole prevents warping of the free end of the legs towards the inside radially, in which the flange and the legs securely in contact with the edge of the attachment hole in the workpiece fix the grommet to the workpiece, in which a threaded section is formed on the inside section of the free end of the legs for receiving the tapping screw and extending towards the inside so the diameter of the threaded through-hole becomes smaller than the outer diameter of the tapping screw, and in which the sections of the legs other than the threaded section are thin and extendable so as to draw the free end closer to the flange when the tapping screw is screwed in and fastened. As a result, the legs are raised towards the surface of the workpiece and more firmly secured.